Archived posting to the Leica Users Group, 2007/02/28
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]this is a leica reply on question somebody had about a 35mm 1.4 asph and focusing problems. more info found on leica user fourm you described that the focus moves back when stopping down the aperture. This effect occurs due to so called spherical aberration. It is an image imperfection that occurs due to the increased refraction that occurs when rays strike the lens near its edge, in comparison with those that strike nearer the center. It?s not possible to remove this aberration completely in the optical design, especially at high speed lenses . Different aberrations need different parameters to correct them. Unfortunately it happens when you improve one aberration (e.G. distortion), that an other aberration increases. At the end it?s a compromise between all kinds of aberrations, size, weight etc. The 35mm f1.4 lens is a well balanced lens regarding all competing parameters. But unfortunately it?s not totally free of any aberrations. It needs to be considered, that stopping down has an effect on the focus plane and probably it needs some exercises in order to get the right feeling for the lens. ...end quote... The email goes on to explain that, since the first lens I sent back (they didn't receive the second one yet) was within the spec (and not at the bottom of it either) I might want to have it sent back to me. Now I have to trust these good and helpful folk but I have replied that if the 35mm Cron does not exhibit this I might have that instead. I have also observed that the PDF technical spec for the 35 1.4 which I downloaded from their website appears to contradict the email in that the very detailed DOF charts show a one metre point of focus remaining within focus throughout the aperture range. I also observed that it might be helpful to let potential purchasers know in advance of purchase that this design constraint exists and that they will need to 'get the right feeling for the lens'. it seems that with M8 alot more is visible and hence all these situations . some of ziess lenses appear on paper to be more corrected than leica's On 2/28/07, G Hopkinson <hoppyman@bigpond.net.au> wrote: > All of this focus problem discussion has me nervous. Are you folks saying > that the body to lens cam interface arrangement needs > adjustment? Or the rangefinder is adjusted? Is the issue always in fact > there, but only noticeable where there is extremely shallow > DoF? Would not any adjustment of the body then degrade the accuracy with > other lenses? > I looked at this when first trying the 50 summilux asph. I certainly miss > focused when I didn't expect to. (this is wide open or > nearly so and within 2 meters, so DoF WAS narrow)The zone of acceptable > focus in my instance seems more forward of the focus point > than I expected. However careful measurement from film plane shows the > rangefinder and lens in exact agreement at 1 meter and > infinity. Shots with a focus test chart also appear correct. > > Cheers, the narrowly focused Hoppy > > -----Original Message----- > FSubject: RE: [Leica] Leica USA service/repair question > > No. The body worked perfectly on anything I threw on it, after Sherry > worked her wonders...... Before, stuff was out of focus > with the 75 Lux, no matter what I did. > > To be fair, the other lenses were the 50 Lux, 35 Lux, etc. Nothing like a > 90 AA or 135/3.4 at that time that might have shown some > issues...... > > Frank Filippone > red735i@earthlink.net > > > did this mess up the other lenses that you did not send in > > > > > > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information > -- ------------------------------------- regards, mehrdad